I agree with the majority - YABU.
I know you say you’ve asked your cousin - but have you had a proper (calm) chat about it or was it an off the cuff question and she’s just quickly replied.
There could be many genuine reasons she’s not asked you - it might be cost related, space restrictions (small church which might look odd with loads of bridesmaids), she might be going the traditional route and if you are married and have a child then you wouldn’t qualify, or it might be she’s including you by way of your son. I’d also question why, if she’s your closest friend, she doesn’t have any relationship with your son. Seems odd to have kept them separate.
I know you disagree that it’s nice, but I’d rather my daughter was a flower girl than me have to be a BM. She’d look so cute, I’d have lovely photos of her dressed up etc... I have some fab shots of me as a kid and I was spoiled rotten by other guests all day when I was a flower girl. I loved it and I was only 4 ish.
While you’ve not gone into whatever you think the reason is that you’ve not been asked, you seem pretty certain it’s not the reason she’s given. You clearly have a low opinion of both yourself and your friend if you think she’s not asked you for some vacuous, superficial reason. I’d doubt you’re that close if that’s the case. Why would you think the worst of your closest friend ?
I’m in two minds what I’d recommend you do. I think ultimately, it’s her wedding and her choice. People love to get offended over weddings. But you’ve had your wedding and you made choices you were happy with - so let her do the same.
Why would you make her feel bad or awkward about her choice, particularly if you’re such good friends ?
My other thought would be to have a grown up, calm conversation with her about it. However, I suspect she’ll then fall out with you for making her feel awful about her choice, thinking the worst of her etc... I can’t see it would end well.
I would just shut up about the BM issue.
Either accept the offer for your son, or if not politely decline the page boy request (just saying he’s too young / unreliable that the minute and you’d be worried about him not enjoying it).
I do have a bit of sympathy for your feelings and I can see that you’re a bit hurt. But it’s one of those situations where I don’t think you logically have much right to be, it’s more of a heart over head thing. But I do think you have to let it go or you’ll ruin this friendship.